Effect of cultivation on chemical characteristics and respiratory activity of crusting soil from Mazowe (Zimbabwe)

Clearing and cultivation in crusting soils from Mazowe (Zimbabwe) has lead to severe changes in most physico-chemical characteristics related to the concentration and distribution patterns of plant nutrients and to the total amount of soil organic matter. Nevertheless, the concentration of the different humus fractions showed lower intensity changes, as did the mineralization rates of the organic matter. The most significant effects of cultivation on the soil chemical characteristics coincided with those considered to favor clay dispersion and crusting phenomena, including generalized desaturation of the exchange complex and losses of divalent ions with a potential bridging effect between soil particles. Concerning the soil organic matter, the humic acid tended to concentrate in the cultivated soils as a probable consequence of selective biodegradation of the other humic fractions. The composition and activity of soil humus suggest low-performance organo-mineral interactions: in these soils the active turnover of the plant wastes is not regulated by intense physico-chemical interactions with the soil mineral fractions, or by physical encapsulation of organic particles. In consequence, the mineralization rates were relatively constant in the soils studied and unrelated to soil organic matter concentration The results suggests that there is a possibility to revert the early degradation stages of these soils through a rational management of suitable amounts of crop wastes.